The question of whether an island can truly sink has a definitive scientific answer: yes, they can, but the process is rarely the sudden vanishing act of fiction. Islands are classified as either continental (extensions of a landmass) or oceanic (volcanic or coral structures built up from the seafloor). The submergence of these landforms is a complex interplay of geology, oceanography, and climate, typically unfolding over immense timescales. The most common reasons involve a change in the island’s relative position to the sea surface, driven by powerful Earth processes.
Gradual Processes of Submergence
Submergence often occurs when the landmass itself slowly subsides, effectively lowering the island relative to a stable sea level. This phenomenon is driven by tectonic subsidence, where an island near a subduction zone is forced downward by the movement of tectonic plates. In the Torres Islands of Vanuatu, this slow compression has been measured, contributing to a relative sea-level rise of over 9 millimeters per year in some periods.
A different sinking mechanism is isostatic adjustment, especially relevant for older volcanic oceanic islands. As the massive volcanic edifice moves away from its hot spot origin, the underlying oceanic crust cools, contracts, and becomes denser. This causes the entire seafloor section, including the island, to sink—a process Charles Darwin theorized to explain the formation of coral atolls. Furthermore, the weight of the island itself can cause the lithosphere to flex downward, adding to the loss of elevation.
The third factor is eustatic sea-level rise, a global increase in the volume of ocean water independent of land movement. This rise is caused by two effects of global warming: the thermal expansion of seawater and the addition of meltwater from ice sheets and glaciers. Since 1992, the global average rate of eustatic rise has been measured at approximately 3.2 millimeters per year. This increase disproportionately threatens low-lying landforms, such as coral atolls in nations like Kiribati and Tuvalu, which possess elevations barely above the current high-tide line.
Physical Disintegration by Environmental Forces
Islands can disappear not by sinking whole, but by being physically worn away by external environmental forces. Coastal erosion is a process where wave action, strong currents, and storms relentlessly strip away beach material and soft rock. This disintegration is rapid on islands composed of unconsolidated sand or softer sedimentary rock, which offer little resistance to the ocean’s energy.
The threat of physical loss is amplified by the degradation of natural coastal defenses, such as coral reefs. Healthy reefs act as submerged breakwaters, absorbing up to 97% of incident wave energy before it reaches the shore. When reefs are damaged by bleaching, ocean acidification, or disease, their ability to dissipate wave energy diminishes significantly. This loss exposes the island’s coastline to the full force of the ocean, accelerating coastal erosion. In some areas, reef degradation has led to seafloor erosion, increasing water depths more rapidly than sea-level rise alone.
Catastrophic Volcanic Collapse
While most island submergence is slow, volcanic islands can experience a sudden loss of elevation through catastrophic collapse. This mechanism is limited to large, active volcanoes and is initiated by the rapid withdrawal or eruption of magma from the chamber beneath the island. When a large volume of magma is ejected, the chamber roof loses structural support, leading to an inward collapse of the overlying landmass.
This process forms a caldera, a large, cauldron-like depression that can be dozens of kilometers wide. The collapses that formed the calderas of Krakatoa (1883) and Hunga Tonga–Hunga Ha‘apai (2022) are examples of such events, where a substantial portion of the island instantaneously dropped below sea level. Another sudden event is a flank collapse, where an unstable section of the volcano’s side detaches and slides into the ocean. This results in the rapid loss of landmass and can trigger powerful tsunamis, representing the most immediate way a volcanic island can sink.